The present invention relates generally to a semiconductor circuit, and more particularly to an MOS integrated circuit including a bias-voltage generator mounted on the substrate thereof, with the bias voltage being applied to this substrate.
Generally, an MOS integrated circuit is provided with a bias-voltage generator so as to establish and maintain the desired optimum circuit operation of each of the semiconductor devices, such as FETs (Field Effect Transistors), which constitute the MOS integrated circuit. For instance, if the threshold voltage is below a specified or nominal level, the semiconductor device is unstable in operation, as it may be improperly rendered conductive by a noise signal, and is thus particularly unsuitable for use in logic circuits. Contrary to the above, if the threshold voltage is greater than the desired level, the speed of the operation of the semiconductor device is reduced, and the semiconductor device may fail to be turned on when an input signal is applied to the control electrode. Thus, the level of the threshold voltage must be maintained at a desired optimum level. The maintenance of the level of the threshold voltage at a desired optimum level is achieved by a bias-voltage generator which functions to absorb the so-called substrate current (I.sub.BB) which flows across the substrate.
In view of the operation thereof, MOS integrated circuits are generally classified into two types of circuits: first, a static-type circuit; and second, a dynamic-type circuit. In the static-type circuit, for example a static-type storage device, there is no limit with respect to the operating frequency. This is because such a static-type storage device operates based not on the stray capacitors accommodated therein, but on a static current supplied from a power supply. In contrast there is a lower limit with respect to the operating frequency of dynamic-type circuits, for example dynamic-type storage devices. This is because such a dynamic-type storage device is operated by using electric charges stored in stray capacitors, which electric charges are discharged therefrom with some RC time constant. Generally, in the former static-type circuit, since this circuit is operated by using said static current, the magnitude of said substrate current does not vary and is held at a constant level. In the latter dynamic-type circuit, however, since this circuit is operated by using said electric charges stored in stray capacitors, the magnitude of the substrate current (I.sub.BB) varies in proportion to the operating frequency thereof. That is, when the operating frequency becomes low, the magnitude of the current (I.sub.BB) is low. Contrary to this, when the operating frequency becomes high, the magnitude of the current (I.sub.BB) is high. Thus, since the operating frequency usually varies within a wide range of frequencies, the bias-voltage generator must be designed so as to have a capability for absorbing the substrate current (I.sub.BB) having a maximum allowed magnitude level, which current (I.sub.BB) will flow when the circuit operates with the highest operating frequency. Therefore, such a bias-voltage generator always consumes relatively high power, even when the circuit operates at a relatively low operating frequency. This fact is the shortcoming of a typical bias-voltage generator.
For the purpose of overcoming the above mentioned shortcoming, an improved bias-voltage generator has been proposed in, for example, U.S. Pat. No. 3,806,741, entitled SELF-BIASING TECHNIQUE FOR MOS SUBSTRATE VOLTAGE, by Frederick J. Smith. In the improved bias-voltage generator, this bias-voltage generator can absorb the substrate current (I.sub.BB) in such a manner that the magnitude thereof varies in accordance with the variation of the operating frequency of the dynamic-type circuit. In other words, the magnitude of the current (I.sub.BB) always matches the operating frequency, thereby the aforesaid shortcoming of the typical bias-voltage generator can be overcome.
However, the improved bias-voltage generator has a disadvantage in that this bias-voltage generator cannot absorb the full substrate current (I.sub.BB) when the dynamic-type circuit operates at a very low operating frequency, due to the presence of the so-called junction leak current, the magnitude thereof not being proportional to the variation of the operating frequency. In other words, since the bias-voltage generator functions to absorb merely the substrate current (I.sub.BB) having a very low magnitude at a time when the operating frequency is relatively low, this bias-voltage generator cannot absorb the junction leak current, because the magnitude of the substrate leak current is higher than the magnitude of the substrate current (I.sub.BB) to be absorbed by this bias-voltage generator.